Static shielding of transformer windings



Sept. 22, 1959 KENTARO KURITA 2,905,911

STATIC SHIELDING OF TRANSFORMER WINDINGS 2 Sheets-Sheet 1 Filed June 8, 1954 FIG.

m 5 a 7 x 6 0 E w Q 2 Q Q Q f m r 4 4 a 6 Q 8 Q Q g a 4 g Q Q Q Q INVENTOR M M Kentaro Kurita, Hitachi City, Japan, assignor to Hitachi Limited, Tokyo, Japan Application June 8, 1954, Serial No. 435,310

4 Claims. (Cl. 336-70) This invention relates to arrangements of static shielding transformer windings, and more particularly to means for shielding transformer windings against steep wave front voltages applied to the windings.

The invention is characterized in that one or more shielding conductors are inserted between adjacent coil turns of the disc coil-s of a power transformer and said shield conductors provided for different coil layers are electrically connected with each other or with one of the coil conductors.

The principal object of this invention is to provide a positive shield means which can considerably reduce the volume and cost of known electrostatic shielding devices.

It is known that the initial potential distribution in a transformer when a lightning stroke or other peak voltage of steep wave front appears at a terminal of the transformer is determined by the ratio of distributed capacities, i.e., the ratio of the capacitance of coils to earth and the mutual capacitance between coils. This distribution is not linear, but it exhibits a particularly steep potential gradient at the high tension terminal of the winding, which is liable to puncture the insulation of the winding. In order to prevent damage, various contrivances have heretofore been made to level off the steep potential gradient by providing electrostatic shields for the windings. Known shields, however, result in an un avoidable increase in the cost and volume of the transformer.

According to this invention, an etiective electrostatic shielding means can be provided for a power transformer without substantially increasing the cost and size of transformer.

For a better understanding of the nature of this invention, reference should be had to the following description in conjunction with the accompanying drawing, wherein:

Fig. 1 is a diagrammatic sectional view of transformer windings illustrating an embodiment of this invention;

Figs. 2 to 5 are diagrammatic views similar to Fig. 1 illustrating modified embodiments of this invention;

Fig. 6 is a diagrammatic representation of an equiva-' lent circuit of a non-shielded transformer, and

tion, in which insulated conductors are used in place of shield conductors 9 to :12 in Fig. 1 and also insulated conductors 15, 16, 17 and 18 are wound on the outside of each end turn 1, 3, 5 and 7 in each layer of disc wound coils, and each pair of the conductors 15, 16 and 17, 18 respectively are electrically connected with each other.

Fig. 4 illustrates a further modified embodiment of this invention, in which shield conductors 23, 24, 25 and- 26 are wound between each turn 2, 4, 6 and 8 as shown in Fig. 3 and that of the turns 19, 20, 21 and 22 which are located adjacent to the inside of the former turns, respectively. If desired, shield conductors may also be wound between the inner turns.

Fig. 5 illustrates further modified connections of shield conductors wound according to this invention, wherein the shield conductors 9, 23, 23 etc. wound on the. outermost and between successive turns respectively of a disc coil layer are connected together with each other and to the coil conductor 3, preferably the outermost turn of the adjacent layer. Similarly the shield condoctors '16, 10, 24, 24' etc. of the lower layer are connected together with each other and to the coil conductor '1, and also the shield conductors 17, 11, 25, 25' etc. are connected together with each other and to the coil conductor 7. The shield conductors 18, 12, 26, 26' etc. are connected together with each other and to the coil conductor 5. 7

Either of bare or insulated shield conductors is selected according to the insulating strength of the insulation covering of conductors, for example, whether the dielectric strength of insulation covering of the conductors 1 and 3 can resist the potential difference between these conductors or not. In disc coils having winding space approximately as shown in these figures, it is sufiicient in practice to use a thin copper strip having thickness of I about 0.8 mm. or which is provided with relatively thin rangement' may insulation coveringso that the width of each disc coil layer required for receiving the shield conductor is not substantially increased. Though in figures referred to above, several coil layers adjacent to the high tension terminal are shown, it should be noted that a similiar arbe provided for lower potential coil layers. i

The operation and effect of the electrostatic shield arrangement according tothis invention will next be explained. Considering an ordinary transformer which has no shield conductors 9, 10, 1 1 and 12 as are provided in the embodiment of this invention shown in Fig. 1, the potential distribution in the transformer windings at the moment when a surge voltage of steep wave front has arrived at the high tension terminal 13 may be determined by the condition of distribution of the capacitance to Fig. 7 shows an equivalent circuit of transformer windings, to which the shield is applied according to this invention.

Referring now to Fig. '1 of the drawing, the reference numerals 1 to 8 designate conductors in each coil of a power transformer. 9 to 12 represent sheet conductors wound between each pair of turns 1 and 2, 3 and 4, 5 and 6, and 7 and 8 respectively; 13 a high tension terminal, '14 a core. The shield conductor-s 9 and 10, and 11 and 12 are electrically connected.

Fig. 2 represents a modified embodiment of this invention, in which the shield conductors 9 and 11 in Fig. 1 are dispensed with, while the shield conductors 10 and 12 are electrically connected toa conductor of turns 1 and 5 respectively and all others are the same as in Fig. 1.

Fig. 3 represents another embodiment of this invenearth and mutual capacitance of the windings. alent circuit is shown in Fig. 6, wherein C, a capacitance to earth of the innermost turn of a disc coil in each layer, C the capacitance to earth of the outermost turn of a disc coil in each layer, K, a capacitance between adjacent turns, and K a capacitance between opposite turns of the upper and lower layers in a set of disc coil layers (usually consisting of two layers arranged in the upper and lower positions as shown in the drawing). a

On the other hand, the equivalent circuit for the ar rangement shown in Fig. 1, wherein the shield conductors 9 to 12 are provided, will be as shown in Fig. 7. In this figure, K represents a capacitance between a shield conductor, such as 9 and a coil conductor adjacent to the shield conductor, such as 1 0r 2.

in ordinary power transformers having no provision of electrostatic shielding for windings, the earth potential An equivrepresents V of different parts of windings at the moment when'ag high voltage surge has arrived at the high tension terminal or the transformer winding is expressed by the following formula as is well known:

In, case of one terminal directly grounded j *sinh cx(,1-X) V E sinh. a In case of one terminal opencirc'uited' cosh 04(1, X) cosh 04 wherein E represents a voltage at aterminal of the winding,

X'-the measured distance between coil layers assuming V a C"capacitance of windings to earth,

K-series capacitance between the high tension terminalandthe neutral terminal. a

shield conductors 9, I15, 23 etc. according to this invention enables increasing the electrostatic capacity substantially between conductors land 3. It is found by measurement that the value of a in each of the above embodiments was reduced to about /3 to of that which was obtainable in known devices.

In conventional devices, shield conductors 15, 16 etc. as shown in Fig. -3 were applied to the outside of end turns l,'2 etc. in each of the coil layers only so that the increase of series capacity per coil layer is relatively small and it is necessary toshield the hightensionterminal and successive several coils with the same potential conductors; v p v H In such a case, a voltage difference of a large percent of total voltage appears between-coil conductors of lower potential and the shield conductors, and thus the thickness of insulation covering of the shield conductors should be considerably increasedin order to resist this magnitude of voltage. The increase in thickness of insulation reducesadditional capacitance between coil conductors, and to compensate for this the connection of shield con ductors. must be extended further to the coil layers of lower potential. This requires a further strengthening It will be apparent from the above formulaethatthe i ofthe insulation of the shield conductors, thereby further; increasing the. size of the shielded coil, which attains a large sectional width; Such bulky shielded windings which project within high tension field from the outer periphery of transformer windings cause an increase in (a) The total series capacitance between the coil conductors l and 3 in Fig. '6 will be series circuit only in Fig. 7 consisting of K and theupper part K inparallel thereto and the. lower part K of series circuit that the series capacitance between conductors 1 and 3 will be increased. r (b) Coil turns 2 and 4 and all other turns -inside or these turns are shielded by the shield conductors '9 and 10 so that the resultant electrostatic capacity is connected in series between the electrostatic capacity between these shield conductors 1 and 3, and the capacitances to earth. C andC and accordingly the capacitance to'earthbetween the turns 1 and 3 and the core 14 will'be more. reduced than when the capacitance to earth consists of Ci and C only. Such an effect could never be obtained by any known electrostatic shield device.v

In the foregoing description, a twin coil layer consisting of an upper 'disc coil layer which comprises turns 1,, 2 etc., and a lower disc coil layer, which comprises turns 3, .4 etc. was taken for the explanation, yet the same applies to the other twin coil. layers. Thus the series capacitance K between the high tension terminal 13 and the neutral point is increased and the capacitance to earth C decreases resulting in the reduction ofthe value or on, thereby making the initial potential distribution coincide with or nearer to the final potential'distributidn.

In the" embodiment shown in Fig, 2, the turns 1fa'nd'9' in Fig. 7 are short-circuited to eliminate the upper. so. that the capacitance between coils 1 and 3 is increased;

By providing further shield conductors 15, 16,717,,18 etc. to, the outside of turns 1, 3, 5, 7 etc. as shown in Fig. 3; the series capacity K between terminals can. be further increased and the value of a. canv be reduced further.

The provision of additional shi ld "conductors between coils inside of shield conductors 9; 10 etc; as shown in Fig. 4 is effective to suppress local oscillation.

In Fig; 4, if the coil' conductor 2 is connected to-the high tension terminal 13, that is, if the coil condiic toi l in.Fig. 3 is constituted by split conductors I and 2' 1n 5 the volume of transformers and also cause troubles in mechanically supporting them. Moreover, this complicates connection between conductors. Thus, an ordinary shieldedv transformer often costs several times the cost of non-shielded transformers of the same capacity.

vAs-described in the foregoing, the shield conductors according to thisinvention are sufiicient with the use of athin copper strip of a few millimeters thickness and with an insulation covering of a few millimeters thickness so that .the increase in volume of coil layers is substantially negligible. Such a strip may be wound together with the turn. in the samewinding operation without requiring any other supporting means and it is a surprising advance that the cost necessary forv providing a shield device can bereducedto afew percent of that of known devices.

What I. claim is:. I

1. An electrostatic shielding device for a transformer comprisingan iron'core and a plurality of pairs of disk coil layers. each comprising a plurality of turns of insulated conductors wound around said core concentrically thereto, said electrostatic shielding device comprising a pluralityof shield conductors: wound between at least the two outermost adjacent turns along the winding direction of more than one pair of said disk coil layers, each' or said shield conductors in one coil layer being electrically connected with at least one of the conductors in the other coil layer belonging to the same pair.

2; An electrostatic shielding device for a power transformer. comprising an iron core and a plurality of pairs of disk coillayers each comprising a plurality of turns of insulated conductors wound around said core concentrically thereto, said electrostatic shielding device comprising a plurality of shield conductors wound to.- gether with and between at least the two outermost adjacent turns of more than one pair of said disk coil layers, a shield. conductor. in one coil layer of a pair being electrically connected with one of said insulated conductors inthe other. coillayer of the same pair.

3. An'electrostatic.shielding device for a power transformer comprising an iron core and a plurality of pairs ofdiskrcoil layers each comprising a plurality of turns of insulated conductors wound aroundv said core con: centrically thereto, said electrostatic shielding device comprising a plurality of shield conductors wound together. with and. between at least the two outermost adjacenf-tui'ns of morethan one pairof said disk coil layers,

each of said shield conductors in one coil layer of a pair being electrically connected with one of said shield conductors in the other coil layer of the same pair.

4. An electrostatic shielding device for a power transformer having an iron core and a plurality of pairs of disk coil layers consisting of a plurality of turns of insulated conductors wound around said core concentrically thereto, said electrostatic shielding device comprising a plurality of shield conductors, each being wound between the outermost turn and the successive inner turns and outside of the outermost turn in each disk coil layer of a pair, said shield conductors being electrically con- 6 nected together with each other and with an insulated conductor in the adjacent coil layer of the same pair.

References Cited in the file of this patent UNITED STATES PATENTS 2,279,027 Weed et a1 Apr. 7, 1942 2,279,028 Weed Apr. 7, 1942 FOREIGN PATENTS 481,233 Great Britain Mar. 8, 1938 500,775 Great Britain Feb. 15, 1939 

